1. Field of the Invention
This invention relates to an organic fluorescent whitening pigment excellentin hiding power and more particularly, it is concerned with an organic fluorescent whitening pigment which can be utilized as coating agents for papers, coloring matters for aqueous or oily inks and inkjets, pigment components, for paints, etc., and which is capable of exhibiting excellent properties when applied to a use, in particular, needing high hiding power as well as high whiteness, for example, a coating agent for surface coating of a paper such as photographic printing paper.
2. Description of the Prior Art
Up to the present time, as a pigment component of a white pigment, there are mainly used inorganic materials, for example, titanium oxide, calcium carbonate, zinc oxide, clay, kaolin, talc, etc. On the other hand, there are no organic whitening pigment components having hiding power comparable to or more excellent than that of these pigments components. As a white organic pigment, alkylenebismelamine derivatives have been proposed as disclosed in JP-A-6-122674, which, however, are excellent in hiding power but insufficient in whiteness because of having no fluorescence.
White pigments consisting of inorganic materials of the prior art are excellent in hiding power, but when using these pigments for coating papers, for example, the whiteness of coated papers is not sufficient. Thus, in order to complement this insufficiency, various fluorescent whitening agents such as of bistriazinylaminostylbene type are ordinarily used, but this has some limitations, because the fluorescent whitening agents often meet with such a particular phenomenon as concentration quenching, i.e. that even if using in a concentration of more than certain value, not only more whitening effect cannot be obtained, but also a yellowish hue is rather given. Accordingly, the fluorescent whitening agent can only be used with a very low concentration.
Generally, inorganic pigment components naturally have large densities and when preparing dispersed compositions therefrom, the pigment components tend to be separated and settled, so that a uniform and stable dispersed composition is hard to be prepared. This is not preferred from the stand-point of the storage stability of the pigment dispersed composition. Further, when such a pigment composition is coated onto a paper, for example, the weight of the coated paper is increased. In addition, the inorganic pigment generally has a disadvantage that its compatibility with binders and other reagents required for preparation of the pigment composition is inferior.
It is an object of the present invention to provide an organic fluorescent whitening pigment having an excellent hiding power as well as high whiteness, that inorganic pigment compositions cannot achieve, whereby the above described problems of the prior art can be solved.
It is another object of the present invention to provide a coating agent for coating a surface of a paper, containing an organic fluorescent whitening pigment having an excellent hiding power as well as high whiteness.
These objects can be attained by an organic fluorescent whitening pigment composition having an excellent hiding power, comprising, as a pigment component a complex salt (compound) represented by the following general formula: 
in which X represents same or different hydrogen atom, methyl, ethyl, methoxy, ethoxy, sulfo group or chlorine atom, R1 represents an alkyl, alkenyl, aralkyl, aryl or cycloalkyl group, and R2, R3 and R4 independently represent hydrogen atom or R1, two or three groups of R2, R3 and R4 being capable of forming a heterocyclic group with nitrogen atom in each case.
The inventors have made various efforts to solve the above described disadvantages of the inorganic white pigments and consequently, have found that a complex salt obtained from some fluorescent whitening agent and quaternary ammonium compound has very excellent properties as an organic white pigment component. The present invention is based on this finding.
The features of the present invention and embodiments thereof will now be summarized below:
(1) An organic fluorescent whitening pigment composition having an excellent hiding power, comprising, as a pigment component, a complex salt (compound) represented by the following General Formula: 
in which X represents same or different hydrogen atom, methyl, ethyl, methoxy, ethoxy, sulfo group or chlorine atom, R1 represents an alkyl, alkenyl, aralkyl, aryl or cycloalkyl group, and R2, R3 and R4 independently represent hydrogen atom or R1, two or three groups of R2, R3 and R4 being capable of forming a heterocyclic group with nitrogen atom in each case.
(2) The organic fluorescent whitening pigment composition having an excellent in hiding power, as described in the above (1), wherein the pigment component is a complex salt (compound) represented by the following formula: 
The above described complex salt of the present invention is a substantially water-insoluble fluorescent complex salt, represented by the following general formula: 
in which X, R1, R2, R3 and R4 have the same meanings as described above, which can be obtained by reaction of a bis-stilbenesulfonic acid derivative or its water-soluble salt, in particular, an alkali metal salt, whose anionic component of the fluorescent whitening agent is generally represented by the following General Formula (I): 
in which X represents same or different hydrogen atom, methyl, ethyl, methoxy, ethoxy, sulfo group or chlorine atom, with a quaternary ammonium compound represented by the following General Formula (II): 
in which Y represents a colorless, non-fluorescent, water-soluble anion or OHxe2x88x92, R1 represents an alkyl, alkenyl, aralkyl, aryl or cycloalkyl group, R2, R3 and R4 independently represent hydrogen atom or R1 or R2, R3 and R4 can form a heterocyclic group with nitrogen atom in each case.
Furthermore, an organic fluorescent whitening pigment excellent in hiding power can be obtained, consisting of this complex salt as a predominant component.
Other embodiments of the present invention are coating compositions for fluorescent papers comprising the above described fluorescent complex salt mixed in coating compositions for papers, or fluorescent white ink compositions for ink jets comprising the above described fluorescent complex salt mixed in white ink compositions for ink jets, processes for the production of the same, methods of using the same, various materials using the same, in particular, coated papers having excellent hiding power and high whiteness, for example, photographic printing papers (photographic papers), etc.
In white pigment compositions for coating papers or plates, clays, calcium carbonate, titnaium oxide, etc. have hitherto been used as a white pigment component. This is mainly due to the excellent hiding power that these white pigments have.
As a result of our studies, it is found that the organic white pigment of the present invention has more excellent hiding power than the inorganic white pigments consisting predominantly of the above described materials. Further, the white pigment of the present invention is so fluorescent that when using the pigment, a higher whiteness that the prior art cannot reach can be obtained. In addition, this fluorescent property does not exhibit concentration quenching that appears in ordinary fluorescent whitening agents (such a peculiar phenomenon to fluorescent whitening agents that when using a fluorescent whitening agent with a concentration higher than a certain value, the whitening effect is rather lowered to render yellowish sometimes). Thus, the white pigment of the present invention can be used in such a high concentration that cannot be considered in the prior art.
The fluorescent whitening pigment component of the present invention has a smaller density, because of being organic, than inorganic pigments. Accordingly, in the case of forming an aqueous dispersion, for example, the pigment component is hard to be settled and the storage stability is good. In addition, a paper coated with the whitening pigment of the present invention has a smaller weight than papers coated with the above described inorganic pigment, thus resulting in reduction of the transporting cost.
The whitening pigment component of the present invention will be illustrated in detail. In the anionic component of General Formula (I), X can be same or different hydrogen atom, methyl, ethyl, methoxy, ethoxy group, chlorine atom or sulfo group. Above all, the most suitable one is represented by the following General Formula (III): 
As the cationic component of General Formula (II), R1 is an alkyl group such as methyl, ethyl, propyl, butyl, dodecyl or octadecyl group, an aralkyl group such as benzyl group, an aryl group such as phenyl group, an alicyclic group such as cyclohexyl group or a five-membered or six-membered heterocyclic group formed with N atom, such as pyrrolidine, piperidine, morpholine or pyridine group, R2, R3 and R4 are independently hydrogen atoms or groups represented by R1 and Y is a quaternary ammonium salt of a halogen atom, preferably chlorine atom, examples of which are represented by the following formulas: 
Preferred examples of the white pigment component according to the present invention are shown in the following Table 1 with specified combinations of groups of General Formulas (I) and (II):
The pigment component of the present invention can, for example, be prepared as follows. A sodium salt of a bis-styryl-biphenyl represented by the foregoing General Formula (III) is added to a suitable amount of water and is then subjected to heating and dissolving at about 60xc2x0 C. with agitation, in which an equivalent or excessive amount of the above described quaternary ammonium compound, dissolved in a suitable amount of water, is poured. Immediately, a white precipitate is deposited and the mixture is heated and stirred at the above described temperature for about 1 hour after the pouring to comlete the reaction. Then, the deposited precipitate is filtered, adequately washed with water and dried at at most 100xc2x0 C. to obtain a pure white powder of the object complex salt.
The thus obtained white pigment component is provided as a white pigment composition in known forms, for example, in the form of a powder, aqueous dispersion, organic solvent dispersion or paste. The organic solvent used herein includes alcohols, esters, ethers, halogen compounds, hydrocarbons, ketones, etc, illustrative of which are butyl alcohol, ethyl acetate, Cellosolve, trichloroethylene, xylene, turpentine oil, solvent naphtha, methyl ethyl ketone, and the like.
The grain diameter range of the pigment component of the present invention is preferably 1 to 2 xcexcm for paper coating, plastic additives, textile printing, etc., 0.5 to 1.0 xcexcm for photographic papers, ink jet inks, etc., 0.5 to 2.0 xcexcm for baking finish and at most 5 xcexcm for synthetic leathers.
Preparation of the pigment compositions in these forms is generally carried out by dry or wet process using known pulverizers, for example, ball mills, sand mills, speed line mills and jet mills. If necessary, the pigment composition is converted into an aqueous dispersion with a surfactant and water using the above described pulverizers and then subjected to spray drying to prepare a powdered dispersion composition.
During the preparation of the above described dispersion composition, a suitable known surfactant is generally used. Depending on the variety of the form or the use of the composition, a suitable one is mainly selected from anionic and nonionic active agents, but in particular, nonionic active agents are preferably used. Furthermore, during or after the preparation of the dispersion composition, commonly used known additives for such pigment compositions, for example, antigelling agents, protective colloids, antiseptics or wetting agents can be added thereto. In the preparation of the dispersion, a suitable pigment concentration is 5 to 50%, preferably 20 to 35% in water or organic solvents.
The important feature of the white pigment composition according to the present invention consists in excellent hiding power as well as high whiteness. For example, when, using a pigment composition shown in the following Preparation Example, an aqueous dispersion composition (pigment concentrations=25%, average grain diameter=0.5 xcexcm) was prepared and another aqueous dispersion composition of titanium dioxide as a pigment component, having the same concentration and same composition were coated and dried under the following conditions:
The whiteness and hiding power of the thus resulting black board paper and Kinshapaper (commercial name) were compared to obtain results as shown in Table 2. In Table 2, the larger is the numerical value, the more excellent is the whiteness. The numerical value of Whiteness (W) shown in Table 2 was obtained from values of Y and Z, measured by means of a color difference meter of SZ-xcexa3 90 type (commercial name), manufactured by Nippon Denshoku KK, by which any five sites of an object were taken and subjected to measurement and an average value was obtained.
As is evident from Table 2, the Kinshapaper and Blackboard Paper having the whitening pigment composition coated according to the present invention are all more excellent in whiteness than those using titanium oxide. This teaches that our whitening pigment composition is also excellent in hiding power.
Another feature of the pigment composition of the present invention consists in that the density of the pigment component composing it is smaller than that of inorganic pigment components because of being organic. This means, for example, that when preparing an aqueous dispersion using the pigment composition of the present invention, grains of the pigment component are hard to be settled and the storage stability of the pigment component is improved. The present pigment composition-coated paper has a smaller weight than papers coated with inorganic pigment-compositions, resulting in reduction of transporting costs and mailing costs. Comparison of the densities of the pigment composition of the present invention, for example, represented by the following structural formula: 
and inorganic pigment components is summarized below:
As apparent from Table 3, the pigment component of the present invention has a density of at most xc2xd times as large as clay having the smallest density of inorganic pigment components. The densities shown in Table 3 are measured by a method using a specific gravity bottle of the solid specific gravity measuremnt methods according to JIS Z 8807.
Another advantage of the organic pigment component of the present invention is that it is excellent in miscibility with various binders such as gelatin, styrene-butadiene type, polyvinyl acetate type, acryl-styrene type (co-)-polymers, acrylic acid esters polymers, etc., gelling inhibitors, protective colloids and preservatives. Thus, the pigment components of the present invention are suitable as a base for coating agents of surface-coated papers, in particular, photographic printing papers or coated papers.